I am not sure if moonflowers are as aggressive as morning glories but I would say it is probably not the best mix. I'm sure there are people who have combined them successfully. but in my experience morning glories are way too prolific and would totally cover your clematis by fall. It might be OK with a really vigorous clematis.

I planted both Henryi and Heavenly Blue Morning Glory's on the same Obeisk last summer and although it really looked pretty for a while, it eventually became a mass of tangled vines. The only good point was that Heavenly Blue Clematis blooms pretty late in the summer.

Linda